U.S. patent application number 12/554099 was filed with the patent office on 2010-03-11 for solar cell and method of manufacturing the same.
This patent application is currently assigned to Electronics and Telecommunications Research Institute. Invention is credited to Hyun Tak KIM, Sang Hoon KIM, JungWook LIM, Sun Jin YUN.
Application Number | 20100059119 12/554099 |
Document ID | / |
Family ID | 41798173 |
Filed Date | 2010-03-11 |
United States Patent
Application |
20100059119 |
Kind Code |
A1 |
YUN; Sun Jin ; et
al. |
March 11, 2010 |
SOLAR CELL AND METHOD OF MANUFACTURING THE SAME
Abstract
Provided are a solar cell and a method of manufacturing the
same. The solar cell includes a substrate; and a light-absorbing
layer formed below the substrate and comprising a plurality of
semiconductor layers which comprise Si or SiGe and have different
Ge composition ratios. According to the present invention, stress
and crystal defects that may occur by sudden changes of the
composition of Ge can be minimized, and a more efficient solar cell
can be fabricated.
Inventors: |
YUN; Sun Jin; (Daejeon-City,
KR) ; LIM; JungWook; (Daejeon-City, KR) ; KIM;
Sang Hoon; (Seoul, KR) ; KIM; Hyun Tak;
(Daejeon-City, KR) |
Correspondence
Address: |
RABIN & Berdo, PC
1101 14TH STREET, NW, SUITE 500
WASHINGTON
DC
20005
US
|
Assignee: |
Electronics and Telecommunications
Research Institute
Daejeon City
KR
|
Family ID: |
41798173 |
Appl. No.: |
12/554099 |
Filed: |
September 4, 2009 |
Current U.S.
Class: |
136/261 ;
257/E21.09; 438/73 |
Current CPC
Class: |
H01L 21/0245 20130101;
H01L 31/1812 20130101; H01L 21/0251 20130101; H01L 21/02532
20130101; H01L 31/065 20130101; Y02E 10/548 20130101; H01L 21/02381
20130101; H01L 31/075 20130101 |
Class at
Publication: |
136/261 ; 438/73;
257/E21.09 |
International
Class: |
H01L 31/00 20060101
H01L031/00; H01L 21/20 20060101 H01L021/20 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 9, 2008 |
KR |
10-2008-0088893 |
Dec 18, 2008 |
KR |
10-2008-0129395 |
Claims
1. A solar cell comprising: a substrate having an electrode layer;
and a light-absorbing layer formed on the substrate and comprising
a plurality of semiconductor layers which comprise Si or SiGe and
have different Ge composition ratios, wherein the Ge composition
ratio of the light-absorbing layer varies so as to correspond to a
Ge composition gradient according to the distance between the
light-absorbing layer and the substrate.
2. The solar cell of claim 1, wherein at least one of the
semiconductor layers has a Ge composition ratio of 0.
3. The solar cell of claim 1, wherein when the substrate is a
transparent substrate that transmits light of a visible light
region, the Ge composition ratio of the semiconductor layers of the
light-absorbing layer increases as the distance between the
semiconductor layer and the substrate increases.
4. The solar cell of claim 3, wherein the light-absorbing layer
comprises a p-type semiconductor layer adjacent to the substrate;
an i-semiconductor layer formed on the p-type semiconductor layer
and having a Ge composition ratio higher than that of the p-type
semiconductor layer; and an n-type semiconductor layer formed on
the i-type semiconductor layer and having a Ge composition ratio
higher than that of the i-type semiconductor layer.
5. The solar cell of claim 4, wherein the i-semiconductor layer
comprises a plurality of SiGe layers having Ge composition ratios
which increase as the distance between the i-semiconductor layer
and the p-type semiconductor layer increases.
6. The solar cell of claim 1, wherein when the substrate is an
opaque substrate that does not transmit light of a visible rays
region, the Ge composition ratio of the semiconductor layers of the
light-absorbing layer decreases as the distance between the
semiconductor layer and the substrate increases.
7. The solar cell of claim 6, wherein the semiconductor layers
comprise an n-type semiconductor layer adjacent to the substrate;
an i-semiconductor layer which is formed over the n-type
semiconductor layer and has a Ge composition ratio lower than that
of the n-type semiconductor layer; and a p-type semiconductor layer
which is formed over the i-type semiconductor layer and has a Ge
composition ratio lower than that of the i-type semiconductor
layer.
8. The solar cell of claim 1, wherein the semiconducting layers
comprise at least one p-i-n unit structure comprising a p-type
semiconductor layer, an n-type semiconductor layer, and an
i-semiconductor layer formed between the p-type semiconductor layer
and the n-type semiconductor layer, and the i-semiconductor layer
has a Ge composition ratio higher than that of the p-type
semiconductor layer comprised in the same p-i-n unit structure and
lower than that of the n-type semiconductor layer comprised in the
same p-i-n unit structure.
9. The solar cell of claim 8, wherein when the p-i-n unit structure
comprises a first unit structure adjacent to the substrate and a
second unit structure formed under the first unit structure, and
when the substrate is a transparent substrate transmitting light of
a visible region, a Ge composition ratio of the i-semiconductor
layer of the second unit structure is higher than that of the
i-semiconductor layer of the first unit structure.
10. A method of manufacturing a solar cell, comprising: loading a
substrate having an electrode layer; depositing a semiconductor
layer comprising Si or SiGe on the substrate; and forming an
i-semiconductor layer by depositing at least one semiconductor
layer having a Ge composition ratio different from that of the
previously deposited semiconductor layer, wherein the Ge
composition ratio of the i-semiconductor layer varies so as to
correspond to a constant composition gradient according to the
distance between the i-semiconductor layer being deposited and the
substrate.
11. The method of claim 10, wherein when the substrate is a
transparent substrate transmitting light of a visible region, the
forming of the light-absorbing layer comprises sequentially
depositing at least one semiconductor layer having a Ge composition
ratio higher than that of the previously deposited semiconductor
layer.
12. The method of claim 10, wherein when the substrate is an opaque
substrate that does not transmit light of a visible region, the
forming of the light-absorbing layer comprises depositing at least
one semiconductor layer having a Ge composition ratio lower than
that of the previously deposited semiconductor layer.
13. The method of claim 10, wherein the forming of the
light-absorbing layer comprises depositing by using at least one of
a digital chemical vapor deposition method, an atmospheric pressure
chemical vapor deposition method, a reduced pressure chemical vapor
deposition method, a plasma enhanced chemical vapor deposition
method, and a reactive thin film deposition method.
Description
CROSS-REFERENCE TO RELATED PATENT APPLICATION
[0001] This application claims the benefit of Korean Patent
Application Nos. 10-2008-0088893, filed on Sep. 9, 2008 and
10-2008-0129395, filed on Dec. 18, 2008, in the Korean Intellectual
Property Office, the disclosure of which is incorporated herein in
its entirety by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a solar cell and a method
of manufacturing the same, and more particularly, to a solar cell
which can minimize stress and crystal defects by forming a
light-absorbing layer so that a Ge composition ratio of a
silicon-germanium (SiGe) thin film solar cell gradually varies.
[0004] 2. Description of the Related Art
[0005] Recently, fossil fuels on the earth are being depleted and
the global environment pollution is getting severe due to the use
of the fossil fuels.
[0006] Thus, different ways of obtaining energy not inducing
pollution must be solved on the earth as soon as possible. Also,
the need for clean, renewable energy that can be replaced with
fossil fuels has increased, and the best renewable energy is solar
light since solar light is available as long as the sun and the
earth exist and does not generate air pollution.
[0007] A solar cell converting solar energy into electricity by the
use of solar light is classified as a solar heat cell and a solar
light cell. The solar heat cell generates steam necessary to rotate
a turbine by using concentrated solar heat (temperature
1000.degree. C.). The solar light cell converts photons into
electrical energy by using the characteristics of a semiconductor.
In general, a solar cell denotes a solar light cell and thus
hereinafter, referred to as a solar cell.
[0008] Solar cells obtain power from a photovoltaic effect. That
is, a p-type semiconductor generating electrical conduction by
holes and an n-type semiconductor generating electrical conduction
by electrons are joined. Then, electrons, holes, and charges are
created by light, and thus, current flows to generate a
photovoltaic effect.
[0009] However, the energy production cost of solar cells is much
higher than that of thermal power generation, waterpower
generation, or nuclear power generation. Therefore, there are needs
for a low production cost, easy installation, and highly efficient
production in relation to an occupying area of the solar cells.
[0010] At present, a solar cell, which has been produced and sold
the most, is a bulk-type silicon (Si) solar cell, which occupies
90% of the solar cell market. However, in the case of the bulk-type
Si solar cell, as an undersupply of Si is anticipated and
lower-priced solar cells are required, the development of a thin
film solar cell is being accelerated.
[0011] The thin film solar cell market occupies 11.6% of the entire
solar cell market in 2007. Thin film solar cells include Si thin
film solar cells formed by applying a Si thin film on a substrate
such as a glass substrate, a metal substrate, or the like, CuInGaSe
(CIGS)-based thin film solar cells, CdTe-based thin film solar
cells, dye sensitized solar cell (DSSC) solar cells, and organic
thin film solar cells, etc. It is anticipated that amorphous Si
thin film solar cells will occupy about 57.5%, CdTe-based thin film
solar cells will occupy about 24.4%, CIS/CIGS-based thin film solar
cells will occupy about 18.1% of the inorganic thin film solar cell
market about in 2010.
[0012] A small quantity of Si is used for the thin film solar cell
as compared to the bulk-type Si solar cell manufactured on a Si
substrate, thus the material cost of the thin film solar cell is
low. The amount of Si used in the amorphous Si solar cells is about
one hundredth of the amount of Si used in the bulk-type Si solar
cell, and thus the manufacturing cost of the thin film solar cell
can be lower even when a larger substrate is used for producing the
same power due to lower efficiency of thin film solar cell than
bulk-Si solar cell.
[0013] FIG. 1 is a table showing the manufacturing cost and light
conversion efficiency according to a type of a silicon solar cell,
wherein the data was obtained from the Korea Institute of Energy
Research in August 2007.
[0014] Referring to FIG. 1, the light conversion efficiency of a
thin film solar cell is about 8%, which is extremely lower than
that of a bulk-type Si solar cell (single crystal is 17%,
polycrystal is 14%).
[0015] A method of decreasing the manufacturing cost and
installation cost of a Si thin film solar cell further is closely
connected with a method of increasing light conversion efficiency
of a solar cell. In particular, in the case of South Korea, for
example, a country having a narrow land area, the area of a solar
cell is the cost itself.
[0016] A method of increasing efficiency of the Si thin film solar
cell includes a method of increasing characteristics such as
crystallinity of a semiconductor layer, which is a light-absorbing
layer, a method of increasing light absorption efficiency by adding
a second material to a Si light-absorbing layer, a method of
decreasing defects at interfaces consuming carriers such as
electrons, holes, etc., and the like.
[0017] Solar light includes ultraviolet (UV) rays, infrared rays as
well as visible rays, which have the highest intensity.
Semiconductors used in a light-absorbing layer of solar light have
bandgaps of more than 1 eV (wavelength <1240 nm). A single
crystal Si thin film, a GaAs thin film, and a CdTe thin film have
bandgaps of 1.12 eV, 1.43 eV, and 1.49 eV, respectively. Since such
thin films cannot effectively absorb light with an infrared region
that is less than 1 eV, materials having a lower bandgap than 1 eV
have attracted attention. One of these materials is Ge, which, in
the case of a single crystal, the bandgap is 0.67 eV. Thus, a SiGe
solar cell is attracting attention over a Si solar cell. When the
Si or Ge films are fabricated to be amorphous films, their bandgaps
increase to 1.4.about.1.9 eV for Si or 1.0.about.1.4 eV for Ge.
[0018] As an attempt in increasing efficiency of a SiGe thin film
solar cell by changing its structure, a technology of introducing a
quantum well structure in a space charge area of a Si p-n junction
diode as an active base area having a high absorbance has been
proposed.
SUMMARY OF THE INVENTION
[0019] The present invention provides a solar cell which can
minimize stress and crystal defects by forming a light-absorbing
layer so that a Ge composition ratio of a silicon-germanium (SiGe)
thin film solar cell gradually varies.
[0020] According to an aspect of the present invention, there is
provided a solar cell including: a substrate; and a light absorbing
layer formed below the substrate deposited by electrode layer and
comprising a plurality of semiconductor layers which include Si or
SiGe and have different Ge composition ratios.
[0021] According to another aspect of the present invention, there
is provided a method of manufacturing a solar cell, including:
loading a substrate; depositing a semiconductor layer including Si
or SiGe on the substrate deposited by electrode layer; and forming
a light-absorbing layer by depositing at least one semiconductor
layer having a Ge composition ratio different from that of the
previously deposited semiconductor layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The above and other features and advantages of the present
invention will become more apparent by describing in detail
exemplary embodiments thereof with reference to the attached
drawings in which:
[0023] FIG. 1 is a table showing the manufacturing cost and light
conversion efficiency according to a type of silicon solar
cell.
[0024] FIG. 2A shows a conventional silicon (Si) thin film solar
cell, and FIG. 2B shows a silicon-germanium (SiGe) thin film solar
cell according to an embodiment of the present invention.
[0025] FIG. 3A shows a SiGe thin film solar cell according to an
embodiment of the present invention, and FIG. 3B shows a
light-absorbing layer of the SiGe thin film solar cell of FIG. 3A
in more detail.
[0026] FIG. 4 shows a bandgap of the SiGe thin film solar cell of
FIG. 3A.
[0027] FIG. 5 shows a solar cell using an opaque substrate,
according to another embodiment of the present invention.
[0028] FIG. 6A shows a conventional SiGe thin film solar cell
having a triple junction structure, and FIG. 6B shows a SiGe thin
film solar cell having a triple junction structure, according to an
embodiment of the present invention.
[0029] FIG. 7 is a flowchart of a method of manufacturing a SiGe
thin film solar cell, according to an embodiment of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0030] The present invention will now be described more fully with
reference to the accompanying drawings, in which exemplary
embodiments of the invention are shown. In the description, the
detailed descriptions of well-known functions and structures may be
omitted so as not to hinder the understanding of the present
invention.
[0031] When one element "includes" one component in the present
invention, it means that the element further includes another
element instead of excluding another element as long as A
description to the contrary does not exist.
[0032] FIG. 2A shows a conventional silicon (Si) thin film solar
cell 200, and FIG. 2B shows a silicon-germanium (SiGe) thin film
solar cell 210 according to an embodiment of the present
invention.
[0033] Referring to FIG. 2A, the Si thin film solar cell 200
includes an anti-reflection film 201, a transparent substrate 202,
transparent electrodes 203 and 207, a p-type Si semiconductor layer
204, an i-type Si semiconductor layer 205, an n-type Si
semiconductor layer 206, and metal electrodes 208. A metal
electrode layer can be used instead of transparent electrode 207
and metal electrode 208. Referring to FIG. 2B, the structure of the
SiGe thin film solar cell 210 according to the present embodiment
of the present invention is similar to that of the Si thin film
solar cell 200 of FIG. 1, but light-absorbing layers 214, 215, and
216 which are respectively formed as a p-type semiconductor layer,
an i-type semiconductor layer, and an n-type semiconductor layer
may include Ge as well as Si. In FIG. 2B, a metal electrode layer
can be used instead of transparent electrode 207 and metal
electrode 208.
[0034] In the Si thin film solar cell 200, unlike a bulk-type Si
solar cell, a light-absorbing layer has a multi-layered structure
including the p-type Si semiconductor layer 204, the intrinsic Si
(i-Si) semiconductor layer 205, and the n-type Si semiconductor
layer 206, similar to the SiGe thin film solar cell 210 of FIG.
2B.
[0035] The transparent substrate 202 may be a glass substrate or a
flexible substrate, such as a polymer film or the like. The
transparent substrate 202 may be replaced with opaque substrates
such as flexible stainless steel film, a metal film, or ceramic
substrate according to its use. When the transparent substrate 202
is an opaque substrate, the Si thin film solar cell 200 may have a
reverse structure to the structure using the transparent substrate
202. In a reverse structure, the transparent electrode 203 is
replaced with metal electrode or conducting multilayer containing a
transparent conducting layer and a metal layer.
[0036] Unlike the current embodiment, an anti-reflection film may
be interposed between the transparent substrate 202 and the
transparent electrode 203, or a buffer layer may be interposed
therebetween in order to improve a characteristic of each
interface.
[0037] In the SiGe thin film solar cell 210, a composition of Ge
varies according to the manufacturing conditions of SiGe, and a
research providing experiment conditions for obtaining the highest
efficiency has been conducted. In the case of prior research, when
the amount of Ge exceeds 20%, it was found that the efficiency of
the SiGe thin film solar cell 210 decreased. That is, as the amount
of Ge increases, many defects around interfaces between a SiGe
layer and a Si layer occur due to lattice mismatch between the Si
and SiGe, and as crystallinity of the thin film solar cell 210
decreases, carriers generated by the defects and the interfaces are
trapped, thereby significantly decreasing conversion
efficiency.
[0038] In the case of a heterojunction such as Si/SiGe, an
interface functions to trap or remove carriers generated by
defects. Therefore, when a SiGe thin film solar cell is
manufactured in an n-Si/Si-graded i-SiGe/p-SiGe (or p-Si/Si-graded
i-SiGe/n-SiGe) structure instead of manufactured in a conventional
n-Si/i-SiGe/p-Si structure, the SiGe thin film solar cell is
manufactured as a homojunction, not as a heterojunction, thus the
n-Si/Si-graded i-SiGe/p-SiGe structure is advantageous. Also, when
the SiGe thin film solar cell is manufactured in an i-SiGe/n-Si or
i-SiGe/p-Si structure, an i-n or i-p interface is formed, thereby
exhibiting a bandgap-narrowing phenomenon. Thus, the
bandgap-narrowing phenomenon in heterojunction has a negative
influence on the efficiency of the SiGe thin film solar cell.
However, when the SiGe thin film solar cell is manufactured only as
a homojunction, such a negative influence is removed, and thus, the
efficiency of the SiGe thin film solar cell is increased.
[0039] FIG. 3A shows a SiGe thin film solar cell 300 according to
an embodiment of the present invention, and FIG. 3B shows a light
absorbing layer of the SiGe thin film solar cell 300 of FIG. 3A in
more detail.
[0040] The SiGe thin film solar cell 300 according to the current
embodiment includes an anti-reflection film 301, a transparent
substrate 302, transparent electrodes 303 and 307, a p-type Si
semiconductor layer 304, an i-type SiGe semiconductor layer 305, an
n-type SiGe semiconductor layer 306, and metal electrodes 308. In
FIG.3A, a single metal electrode layer can be used instead of
transparent electrode 307 and metal electrode 308.
[0041] A light-absorbing layer of the SiGe thin film solar cell 300
includes the p-type Si semiconductor layer 304, the i-SiGe
semiconductor layer 305, and the n-type SiGe semiconductor layer
306. Normally, p-type layer and n-type layer are required to form
p-i-n diode device and behave as conducting layers carrying holes
or electrons, respectively. Among the p-, i-, and n-layers, the
major light absorbing layer is i-SiGe layer. In the light-absorbing
layer, a Ge composition ratio of the SiGe layer disposed in the
closest portion from an incident direction of solar light is lowest
(Si.sub.a1Ge.sub.(1-a1)), and a Ge composition ratio of the SiGe
layer disposed in the farthest portion is highest
(Si.sub.a4Ge.sub.(1-a4)). The a1 is 1, and thus the
(Si.sub.a1Ge.sub.(1-a1)) region may be a Si region. Thus, the
light-absorbing layer should be manufactured so that the Si
composition ratio in SiGe layer varies in the order of
a1>a2>a3>a4. Here, the a1, a2, a3, and a4 denote the
average composition ratio (Si composition in SiGe layer) of their
corresponding layer, and it does not mean that their corresponding
layer includes four layers.
[0042] That is, the Ge composition ratio increases as the distance
between the transparent substrate 302 and the SiGe layer (that is,
the distance between the transparent substrate 302 and the portion
to which solar light is incident) increases.
[0043] According to the current embodiment, a bandgap of a portion
having the smallest amount of Ge is largest, and as the amount of
Ge increases, the bandgap is reduced. In such a structure, since a
heterojunction does not exist, unlike the case where a plurality of
thin films each having a different amount of Ge are manufactured as
a multi-layer, a possibility to trap and remove carriers can be
significantly decreased. The SiGe layer having such gradual and
successive composition gradient may be used in a solar cell having
a multi-junction structure such as a double junction structure, a
triple junction structure, etc., as well as a single junction
structure shown FIG. 3A.
[0044] Here, the multi-junction structure is a structure formed by
repetitively arranging a p-i-n unit structure including a p-type
semiconductor layer, an n-type semiconductor layer, an
intrinsic(i-) semiconductor layer which is interposed between the
p-type semiconductor layer and the n-type semiconductor layer.
[0045] FIG. 4 shows a bandgap of the SiGe thin film solar cell 300
of FIG. 3A.
[0046] Referring to FIG. 4, the size of bandgap gradually decreases
in the order of
E.sub.g(Si)>E.sub.g(a1)>Eg.sub.(a2)>Eg.sub.(a3)>Eg.s-
ub.(a4)>Eg.sub.(b1). For example, the composition of very first
position of i-layer interfaced with p-Si is Si 100% and the Ge
composition of the last position of i-layer interfaced with n-SiGe
layer is the same as n-SiGe layer.
[0047] FIG. 5 shows a solar cell using an opaque substrate 501,
according to another embodiment of the present invention.
[0048] Referring to FIG. 5, the solar cell according to the current
embodiment has a reverse structure to the structure of the solar
cell described in FIG. 3A. That is, the solar cell has a structure
where a metal electrode 502, a reflection film 503, an n-type SiGe
semiconductor layer 504, an i-SiGe semiconductor layer 505, a
p-type Si semiconductor layer 506, a transparent electrode 507, and
a patterned-metal electrode 508 are sequentially formed on the
opaque substrate 501.
[0049] According to another embodiment, the p-type Si semiconductor
layer 506 may be replaced with a p-type SiGe semiconductor layer
having a Ge composition ratio lower than that of the i-SiGe
semiconductor layer 505. Also, the i-SiGe semiconductor layer 505
may be formed so that the composition varies gradually as shown in
FIG. 3B.
[0050] Similarly to the structure of the SiGe thin film solar cell
300 of FIG. 3A, a light-absorbing layer is manufactured so that a
Ge composition ratio of a SiGe layer disposed in the closest
portion from an incident direction of solar light is lowest and a
Ge composition ratio of a SiGe layer disposed in the farthest
portion from an incident direction of solar light is highest.
Accordingly, the n-type SiGe semiconductor layer 504 has a Ge
composition ratio higher than that of the i-SiGe semiconductor
layer 505.
[0051] That is, the SiGe thin film solar cell 300 of FIG. 3A is
manufactured so that as the distance between the transparent
substrate 302 and the SiGe layer increases, the Ge composition
ratio increases. On the other hand, in the current embodiment shown
in FIG. 5, since the position of the opaque substrate 501 is
opposite with respect to the incident direction of solar light, as
the distance between the opaque substrate 501 and the SiGe layer
increases, the Ge composition ratio decreases.
[0052] FIG. 6A shows a conventional SiGe thin film solar cell
having a triple junction structure. FIG. 6B shows a SiGe thin film
solar cell having a triple junction structure, according to an
embodiment of the present invention.
[0053] That is, FIG. 6B shows a structure in which a SiGe layer,
having a composition gradient of which a Ge composition ratio is
gradually increased, is applied to the conventional SiGe thin film
solar cell having a triple junction structure of FIG. 6A.
[0054] Referring to FIG. 6B, the composition of each of the thin
films forming a light-absorbing layer of the SiGe thin film solar
cell may be gradually varied from Si to SiGe. Here, p-type (p-),
intrinsic (i-), n-type (n-) thin films may be formed of SiGe thin
films having different Ge compositions, respectively. In this case,
the SiGe thin film including the smallest amount of Ge may be
disposed in the closest portion from an incident direction of solar
light, and the SiGe thin film including the largest amount of Ge
may be disposed in the farthest portion from an incident direction
of solar light.
[0055] In the SiGe thin film solar cell having a multi-junction
structure of FIG. 6B, the SiGe thin film having the smallest amount
of Ge may be disposed in a close part from an incident direction of
solar light, and the SiGe thin film having the largest amount of Ge
may be disposed in a far portion from an incident direction of
solar light.
[0056] For example, as shown in FIG. 6B, if the SiGe thin film
solar cell is manufactured to have a composition gradient of Ge
from a i-SiGe layer, a1 is 1. That is, the layer corresponding to
is formed as only a Si film at first and then Ge is added to the
layer corresponding to. The SiGe thin film solar cell may be
manufactured so as to satisfy the condition of a1>a2, then an
n-SiGe layer may have a composition gradient of Ge or may be
manufactured as a SiGe layer having a single composition. At least,
the condition of a2>b1 should be satisfied, and a
p-Si.sub.b2Ge.sub.(1-b2) layer to be manufactured next may be
manufactured as a layer having a single composition similar to the
n-Si.sub.b1Ge.sub.(1-b1) layer or as a layer having more
composition of Ge and having a successive gradient of Ge, and the
condition of b1.gtoreq.b2 should be satisfied. Next, the i-SiGe
layer may be manufactured to satisfy the condition of a3>a4, and
then the n-Si.sub.b3Ge.sub.(1-b3) layer may be manufactured as a
SiGe layer having a single composition or having a composition
gradient of Ge, and the condition of b1.gtoreq.b2.gtoreq.b3 should
be satisfied.
[0057] The SiGe thin film solar cell of the current embodiment may
be modified in various ways as follows. The SiGe thin film solar
cell may include only one thin film having a successive composition
gradient of Ge, for example,
p-Si/i-Si/n-Si//p-Si/i-Si/n-Si//p-Si/i-SiGe(graded)/n-SiGe, or may
include a plurality of layers.
[0058] The SiGe thin film having such a composition gradient may be
deposited by chemical vapor deposition, atmospheric
pressure/reduced pressure chemical vapor deposition, plasma
chemical vapor deposition, or the like. Besides, several different
kinds of thin film deposition methods may be used to deposit a thin
film having a composition gradient.
[0059] In the current embodiment, a substrate may be a metal plate,
a metal foil, a polymer substrate, a ceramic substrate, or the
like, as well as glass.
[0060] FIG. 7 is a flowchart of a method of manufacturing a SiGe
thin film solar cell, according to an embodiment of the present
invention. Normally the processes S702, S703, and S704 are carried
out in-situ in one chamber or three chambers in a deposition system
without exposing the substrate to air.
[0061] Referring to FIG. 7, a substrate including an electrode is
loaded (S701), then a p-type semiconductor layer formed of Si or
SiGe is deposited on the substrate (S702).
[0062] At least one semiconductor layer having a Ge composition
ratio different from that of the previously deposited semiconductor
layer is deposited (S703) to form a light-absorbing layer. Here, a
light-absorbing layer may include the semiconductor layer deposited
on the substrate and the at least one semiconductor layer which is
deposited on the substrate.
[0063] Then, n-type semiconductor layer formed of SiGe is deposited
on the substrate (S704).
[0064] According to the current embodiment, when the substrate is a
transparent substrate that transmits light of a visible light
region, at least one semiconductor layer having a Ge composition
ratio higher than that of the previously deposited semiconductor
layer may be sequentially deposited.
[0065] According to the current embodiment, when the substrate is
an opaque substrate that does not transmit a visible light, at
least one semiconductor layer having a Ge composition ratio lower
than that of the previously deposited semiconductor layer may be
sequentially deposited.
[0066] The SiGe thin film according to the current embodiment is
manufactured to have a characteristic in which a Ge composition
ratio varies continuously to correspond to a composition gradient
according to the distance between the substrate and the portion to
which solar light is incident.
[0067] For example, the SiGe thin film may be manufactured so that
the Ge composition ratio of the SiGe thin film gradually increases
or decreases by gradually varying relative composition ratios of
SiH.sub.4, which is a Si precursor, and GeH.sub.4, which is a Ge
precursor, among a reacting gas.
[0068] The above method is just an example, and the SiGe thin film
may be manufactured by other methods. For example, when the SiGe
thin film having a composition gradient is deposited then a new
SiGe thin film is deposited, the relative composition ratios of the
SiH.sub.4 and GeH.sub.4 may be maintained equally with that of the
SiGe thin film, which is previously deposited, in order to allow
the new SiGe thin film to have a constant SiGe composition.
Alternatively, the relative composition ratios of the SiH.sub.4 and
GeH.sub.4 may be deposited to have a different composition from the
SiGe thin film, which is previously deposited, in order to allow
the new SiGe thin film to have a composition gradient.
[0069] Here, the SiH.sub.4 and GeH.sub.4 are just examples of a Si
precursor and a Ge precursor, respectively, and thus different
kinds of Si and Ge precursors may also be used.
[0070] The sequential deposition method according to the current
embodiment may be a digital chemical vapor deposition method, an
atmospheric pressure chemical vapor deposition method, a reduced
pressure chemical vapor deposition method, a plasma enhanced
chemical vapor deposition method, a different type of reactive thin
film deposition method, or the like.
[0071] As such, in the SiGe thin film solar cell, crystal defects
and stress may be minimized by performing a method of gradually
reducing the amount of Ge of the light-absorbing layer, that is,
using a structure having a gradual composition gradient. When a
SiGe layer is manufactured to have a gradual composition gradient,
stress that may occur by sudden changes of a Ge composition can be
reduced, thereby minimizing defects. Also, as the Ge composition
increases, the depth of light absorption decreases, thereby
performing a more efficient light absorption. In order to increase
light conversion efficiency, a thin film having a large bandgap may
be disposed in a close portion from an incident direction of light,
and a thin film having a small bandgap may be disposed in an
opposite direction to the incident direction of light.
[0072] In such a structure, since an interface does not exist
unlike the case where a plurality of thin films each having a
different amount of Ge are manufactured as a multi-layer, a
possibility to trap and remove carriers can be significantly
decreased. The SiGe layer having a gradual composition gradient may
be used in a multi-junction solar cell such as a double junction
solar cell, a triple junction solar cell, or the like, as well as
in a single junction solar cell.
[0073] A light absorbing layer according to current embodiment may
be formed to have a continuous composition gradient. Also, for
convenience of manufacture, the light absorbing layer may be formed
by continuously depositing a plurality of layers having
discontinuous intervals of Ge composition ratios. In this case, an
interface still can be fabricated not to be abrupt, thus
consequently, the same effect as the light absorbing layer which is
formed to have a continuous composition gradient can be
obtained.
[0074] According to the present invention, crystal defects and
stress can be minimized by gradually controlling a Ge composition
ratio of a light-absorbing layer of a solar cell, and a more
effective use of absorbed light can be realized.
[0075] While the present invention has been particularly shown and
described with reference to exemplary embodiments thereof, it will
be understood by those of ordinary skill in the art that various
changes in form and details may be made therein without departing
from the spirit and scope of the present invention as defined by
the following claims.
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